1. Field of the Invention
The present invention relates to a process for an efficient preparation of a mesophase pitch which is suitable for the production of high performance carbon fibers, from a coal tar or a petroleum residual oil. More particularly, the present invention relates to a process for the preparation of a homogeneous mesophase pitch which comprises a continuous thermal treatment of a heavy oil or a pitch which does not contain quinoline insoluble fractions, and preferably contain no or substantially no xylene insoluble fraction, in the presence or absence of an aromatic oil, subsequent distillation or flash distillation of the thermally treated product in a distillation or flash distillation column, recovery of a pitch with extremely low quinoline insoluble contents from the bottom of the distillation or flash distillation column, hydrogenation of the pitch by a continuous thermal treatment in the presence of a hydrogen-donating solvent, distillation or flash distillation of the hydrogenated reaction products in a distillation or flash distillation column and recovery of a hydrogenated pitch from the bottom of the distillation or flash distillation column, and a thermal treatment of the hydrogenated pitch. The mesophase pitch prepared by the process of the present invention is suitable for the production of high performance carbon fibers.
Since high performance carbon fibers are light in weight and show high tensile strength and high modulus of elasticity, they are currently attracting a wide concern as components of composite materials used for airplanes, sports goods, industrial robots, etc., and a great increase in demand is expected.
2. Description of the Prior Art
Heretofore, high performance carbon fibers are mainly polyacrylonitrile (PAN) based carbon fibers which are produced by spinning of PAN, conversion to infusible state in an oxidizing atmosphere, and subsequent carbonization or graphitization under an inert atmosphere. However, the production of PAN-based carbon fibers suffers from several disadvantages. For example, the raw materials of PAN-based carbon fibers are expensive, the carbonization yields are low, and toxic substances are produced during the production. Recently, it has been found that high performance carbon fibers, with properties equal or superior to those of PAN-based carbon fibers, can be produced from a mesophase pitch, without the problems associated with PAN-based fibers.
In the production of high performance carbon fibers from a pitch, it is necessary that the spinning pitch should be a so-called mesophase pitch, which means that it contains mainly mesophase showing an optical anisotropy when examined on a polarized microscope.
The mesophase belongs to a class of liquid crystals which are formed on heating of a heavy oil or a pitch, and it is considered that it shows an optical anisotropy because planar aromatic molecules, formed by thermal polymerization, align themselves in a layered structure. When fibers are produced by melt spinning from such a mesophase pitch, the planar aromatic molecules align themselves along the axis of the fiber by the stress exerted during passage through a nozzle hole, and this aligned structure is maintained throughout the steps of the conversion to an infusible state and carbonization processes, and thus allows production of highly oriented high performance carbon fibers.
As the raw material for the production of such a mesophase pitch, heavy oils such as coal tars, tar by-products from naphtha thermal cracking, tar by-products from gas oil thermal cracking, and decant oils from catalytic cracking, or pitches derived from the heavy oils may be used.
It has been known that such raw materials contain quinoline insoluble fractions like free carbons in coal tars which mean very fine sooty substances with a diameter of 0.1-0.3 microns, or highly polymerized components, and a high quality mesophase pitch with a high degree of orientation cannot be produced from a raw material containing such free carbons, because the layered structure of planar aromatic molecules which are constituent of mesophase is disturbed by deposition of free carbons on the mesophase. Further, a high quality mesophase cannot be produced from a pitch containing highly polymerized components because they are converted to coke-like substances during thermal treatment. Therefore, it is essential that these impurities should be eliminated during one of the steps for the production of a mesophase pitch. However, when a mesophase pitch which has mesophase content of more than 80% is produced by thermal treatment after a simple removal of these impurities and without any other pretreatments, it shows a softening point of higher than 330.degree. C. measured by Mettler method. A spinning temperature of higher than 360.degree. C. is required for spinning of such a pitch. Since this is a temperature range where organic compounds generally decompose, spinning at these temperatures give rise to problems such as breaking off of fibers and lowering of tensile strength.
For example, Japanese Patent Laid-open No. 54(1979)-55625 discloses a process wherein a previously filtered pitch is heated at a temperature range of 380.degree.-430.degree. C., at a residence time range of 5-44 hr while bubbling an inert gas. By this process, however, mesophase pitches obtained have a softening point range of 330.degree.-350.degree. C. And, in a example of the invention, a mesophase pitch having a softening point of 341.degree. C. is spun at a high temperature of 372.degree. C.
Also, Japanese Patent Laid-open No. 59(1984)-164386 discloses a process which comprises heat-treating a pitch at a temperature range of 350.degree.-500.degree. C. to form 10-30 wt% of mesophase in the pitch, removing the formed mesophase by solvent extraction, and heat-treating the solvent soluble pitch thus obtained. By this process, however, softening point of the mesophase pitch shown in the example is higher than 350.degree. C. Accordingly, spinning is conducted at a temperature of higher than 370.degree. C.
Japanese Patent Laid-open No. 58(1983)-136835 discloses a process which is nearly the same as the process of Japanese Patent Laid-open No. 59-(1984)-164386, but it does not disclose about the properties of mesophase pitch or the conditions of spinning.
Further, Japanese Patent Laid-open No. 59(1984)-82417 discloses a process which comprises heat-treating a heavy residual oil having a boling point of higher than 410.degree. C. at atmospheric pressure, removing the formed insoluble fractions by filtration or centrifugation at a temperature range of 200.degree.-350.degree. C., and vacuum distillating the refined pitch at a temperature range of 370.degree.-390.degree. C. In this Japanese Patent Laid-open No. 59(1984)-82417, softening point of a mesophase pitch is not disclosed, however, spinning is conducted at a temperature of higher than 365.degree. C.
Additionally, it is well known that the insoluble fractions formed by the heat treatment of residual oils or pitches contain fine mesophase spheres, and that it is extraordinarily difficult to remove the fine mesophase spheres from the pitches obtained by the heat treatment. Accordingly, removal of the insoluble fractions is not economical and not practical.
A few processes have been proposed which will avoid the disadvantages. For example, Japanese Patent Laid-open Nos. 58(1983)-214531 and 58(1983)-196292 disclose processes for the preparation of a mesophase pitch, which comprise conducting a thermal treatment of a pitch, in the presence of a hydrogen-donating solvent and in the presence or absence of hydrogen gas, and after removal of insoluble fractions formed and the solvent used, further conducting thermal treatment. A mesophase pitch prepared by these processes has a low softening point and excellent spinning properties, and the carbon fibers produced from this pitch have excellent characteristics.
In Japanese Patent Laid-open Nos. 58(1983)-214531 and 58(1983)-196292 mentioned above, however, the hydrogenation of a pitch is carried out by a batch process in an apparatus such as an autoclave. As the hydrogenation of a pitch is generally carried out at a high temperature above 400.degree. C., when heating is applied from the outside of a large apparatus such as an autoclave, the temperature of the wall of the apparatus tends to become considerably high, and the formation of cokes cannot be avoidable. Although hydrogenation of a pitch is very effective for the preparation of a spinning pitch for the production of high performance carbon fibers, when hydrogenation is performed by a batch process, an additional process for removal of quinoline insoluble fractions is required. Moreover, since the hydrogenation is usually carried out under a high pressure, this process requires a high cost in the construction of the apparatus, and temperature control becomes increasingly difficult as the size of the apparatus becomes larger.
Also, U.S. Pat. No. 4,589,975 discloses a process which comprises hydro-treating a pitch in the presence of tetralin, removing insoluble fractions and solvent used, and heat-treating the hydro-treated pitch. In this process, since tetralin acts as a poor solvent, considerable amounts of insoluble fractions are formed in the hydro-treated material. Accordingly, this hydro-treatment can be conducted only by batch procss. As mentioned above, batch process of the hydro-treatment is not economical.
In the circumstances above, development of a process has been desired which reduces the formation of undesirable fractions in each step during the preparation of a pitch for the production of high performance carbon fibers, and requires no means for the removal of the undesirable fractions.
We have proposed processes for the production of pitches suitable for use as the raw material for the production of carbon fibers (Japanese Patent Laid-open Nos. 61(1986)-103989 and 61(1986)-238885). Although these processes are meritorious in many points, we consider that there are still some points to be improved.